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Good Ductwork Design Can Spare You Trouble Down the Line

The importance of proper ductwork design is often evident only when it’s absent. Ducts fulfill their critical function silently, hidden in attics, crawl spaces and inside wall voids. Where ductwork layout and design are performed according to industry standards and quality materials are utilized in fabrication, the homeowner’s not even aware of the ducts. However, this is often not the reality when it comes to residential construction. Ductwork design was often based on a rule-of-thumb, “one size fits all” philosophy that emphasized cost-cutting, not performance and efficiency. Materials utilized in ductwork were not suitable to last the life of the home. Today, many homeowners are painfully aware of the deficiencies of ductwork design every time they open the monthly utility bill. Duct efficiency only slightly above 50 percent is common and, in the average house, at least 20 percent of conditioned air conveyed through ducts leaks out into unconditioned zones instead of cooling or heating living spaces. 

Air Duct Sizing and Installation

Neglecting ductwork design imposes unnecessary costs that persist as long as the ductwork is installed. Conversely, duct efficiencies above 80 percent that result from scientific design are doable with only nominal additional expense. Observing accepted design principles pays off in other ways, too, such as permitting installation of smaller, more efficient HVAC systems at a lower price without sacrificing interior comfort.

Ductwork design performed by an experienced HVAC professional begins with Manual D, the duct sizing software developed by the Air Conditioning Contractors of America. Utilizing information from a cooling and heating load calculation on the residence, Manual D synthesizes factors such as the blower volume and location of the air handler, the total span of the ducts, the internal air friction produced by the ductwork, and the necessary volume of air conveyed to each room for adequate cooling or heating.

Ductwork Requirements

Data generated by Manual D is integrated into an overall design that meets these guidelines:

  1. As much of the total span of ductwork as possible should be installed inside the conditioned zone in the home. Ducts routed through unconditioned areas like attics and crawl spaces are subject to thermal loss from acute temperatures. Where high-efficiency cooling and heating equipment incorporating variable speed blowers is installed, ductwork outside the conditioned envelope should be insulated to the standards of the 2000 International Energy Conservation Code. Due to the lowered airflow generated by variable speed systems, thermal loss in unconditioned zones is more significant.
  2. Building cavities such as voids inside walls, channels between ceiling joists and other preexisting wooden conduits should not be utilized as substitute ductwork. All conditioned air should be conveyed by “hard” duct segments fabricated of metal or fiberglass and mechanically fastened and sealed at all joints. 
  3. In two-story homes or residences with large square footage, consider installing separate HVAC systems for each level or disparate area of the house. Alternatively, a single cooling and heating system may be adapted to split-level homes by incorporating a zoning system with motorized dampers to direct air to zones controlled by independent thermostats.  
  4. Manually operated dampers should be installed at the take-offs where individual room ducts branch off of the main air supply trunk. This allows customizing the air flow to each individual room in order to achieve a neutral air balance in all living spaces, the optimum condition for cooling and heating. 
  5. Shortening the total length of the ductwork at every opportunity reduces internal air friction and makes airflow more consistent to and from all rooms. Installing all supply vents in interior walls closer to the main supply trunk helps keep the span of ductwork shorter.   
  6. The ideal return air system includes a dedicated return vent in every room with a supply duct. Where the ideal is not feasible, a single central air return can be located in a common area like a hallway. To ensure a continuous air path from supply vents to the central return, individual rooms should have air pass-through grilles installed in doors or walls or jumper ducts routed through ceilings to convey supply air from room to room. 

With a network of more than 50 heating and cooling contractors in the Chicagoland area, Comfort24-7.com is your local source of all-season HVAC sales and service. Contact us for more information about optimizing ductwork design to build in maximum indoor comfort and efficiency.

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